Natural gas serves as the energy source for much of the currently generated electricity. To this end, the gas undergoes combustion in a gas turbine which powers an electrical generator. However, the products of combustion leave the gas turbine as an exhaust gas quite high in temperature. In other words, the exhaust gas represents an energy source itself. This energy is captured in a heat recovery steam generator (“HRSG”) that produces superheated steam that powers another electrical generator.
Such exhaust gas includes carbon dioxide and water in the vapor phase, but also includes traces of sulfur in the form of sulfur dioxide and trioxide. Those sulfur compounds, if combined with water, produce sulfuric acid which is highly corrosive. As long as the temperatures of the heating surfaces remain above the acid dew point temperature of the exhaust gas, SO2 and SO3 pass through the HRSG without harmful effects. But if any surface drops to a temperature below the acid dew point temperature, sulfuric acid will condense on that surface and corrode it.
Dew point temperatures vary depending on the fuel that is consumed. For natural gas the temperature of the heating surfaces should not fall below about 140° F. For most fuel oils it should not fall below about 235° F.
Generally, an HRSG comprises a casing having an inlet and an outlet and a succession of heat exchangers—namely a superheater, an evaporator, and a feedwater heater arranged in that order within the casing between the inlet and outlet.
Such heat exchangers for an HRSG can have multiple banks of coils, the last of which in the direction of the gas flow can be a feedwater heater. Surfaces vulnerable to corrosion by sulphuric acid do exist on the feedwater heater. The feedwater heater receives condensate that is derived from low-pressure steam discharged by the steam turbine, and elevates the temperature of the water. Then the warmer water from the feedwater heater flows into one or more evaporators that convert it into saturated steam. That saturated steam flows on to the superheater which converts it into superheated steam. From the superheater, the superheated steam flows to the steam turbine.
In this process, by the time the hot gas reaches the feedwater heater at the back end of the HRSG, its temperature is quite low. However, that temperature should not be so low that acids condense on the heating surfaces of the feedwater heater.
Generally, in the above-discussed process, most HRSGs produce superheated steam at three pressure levels—low pressure (LP), intermediate pressure (IP) and high pressure (HP). Further, an HRSG can have what are termed an LP Evaporator, an HP Economizer, and an IP Economizer. The feedwater heater typically discharges some of the heated feedwater directly into an LP evaporator.
A feedwater heater, or preheater, in a steam generator extracts heat from low temperature gases to increase the temperature of the incoming condensate before it goes off to the LP evaporator, HP economizer, or IP economizer. Multiple methods have been used to increase the temperature of the condensate before it enters any part of the preheater tubes within the gas path (e.g., recirculation pump, external heat exchanger). These methods are used to prevent the exhaust gas temperature from dropping below the acid dew point and causing sulfuric acid corrosion.
Prior systems and methods have been limited in application because the feedwater temperature was not high enough to protect against dew point corrosion of all fuels. The movement of the heat transfer coils to the hotter regions provides for higher differentials in the heat exchanger.
In the present disclosure, an external water-to-water heat exchanger heats the lower temperature inlet condensate with the source of heat being hot water that is exiting the first stage of the feedwater heater. The condensate flow first enters the external heat exchanger. Thereafter preheated condensate leaves the external heat exchanger and enters the feedwater heater. Water energy exiting the preheater is used to preheat the incoming condensate. The present disclosure places a section of a preheater surface into a hotter section of the gas flow, upstream of the LP evaporator, to achieve the beneficial result of increasing source inlet temperature and directly increasing the outlet temperature of the preheated condensate exiting the external heat exchanger. This arrangement allows the use of an external heat exchanger in designs with higher dew points in the cold end. The present system and method can thus create a larger temperature differential in the external water-to-water heat exchanger. This larger temperature differential than present in the prior art, yields a higher outlet temperature and protects the HRSG from cold end condensation corrosion from fuels with higher acid dew points.
The foregoing and other features and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.